Structure–property relation and relevance of beam theories for microtubules: a coupled molecular and continuum mechanics study

Quasi-one-dimensional microtubules (MTs) in cells enjoy high axial rigidity but large transverse flexibility due to the inter-protofilament (PF) sliding. This study aims to explore the structure–property relation for MTs and examine the relevance of the beam theories to their unique features. A molecular structural mechanics (MSM) model was used to identify the origin of the inter-PF sliding and its role in bending and vibration of MTs. The beam models were then fitted to the MSM to reveal how they cope with the distinct mechanical responses induced by the inter-PF sliding. Clear evidence showed that the inter-PF sliding is due to the soft inter-PF bonds and leads to the length-dependent bending stiffness. The Euler beam theory is found to adequately describe MT deformation when the inter-PF sliding is largely prohibited. Nevertheless, neither shear deformation nor the nonlocal effect considered in the ‘more accurate’ beam theories can fully capture the effect of the inter-PF sliding. This reflects the distinct deformation mechanisms between an MT and its equivalent continuous body

Cell morphology governs directional control in swimming bacteria

The ability to rapidly detect and track nutrient gradients is key to the ecological success of motile bacteria in aquatic systems. Consequently, bacteria have evolved a number of chemotactic strategies that consist of sequences of straight runs and reorientations. Theoretically, both phases are affected by fluid drag and Brownian motion, which are themselves governed by cell geometry. Here, we experimentally explore the effect of cell length on control of swimming direction. We subjected Escherichia coli to an antibiotic to obtain motile cells of different lengths, and characterized their swimming patterns in a homogeneous medium. As cells elongated, angles between runs became smaller, forcing a change from a run-and-tumble to a run-and-stop/reverse pattern. Our results show that changes in the motility pattern of microorganisms can be induced by simple morphological variation, and raise the possibility that changes in swimming pattern may be triggered by both morphological plasticity and selection on morphology

Burgerlike ongehoorsaamheid : 'n teologiese besinning oor die kerklike debat

Proefskrif (M. Th.) -- Universiteit van Stellenbosch, 1988.Full text to be digitised and attached to bibliographic record

Versoening en die versoeningsproses : 'n teologiese besinning oor die Suid-Afrikaanse debat

Proefskrif (D. Th.) -- Universiteit van Stellenbosch, 1992.Een kopie mikrofiche.Full text to be digitised and attached to bibliographic record

Feasibility of high pressure freezing with freeze substitution after long-term storage in chemical fixatives

Fixation of biological samples is an important process especially related to histological and ultrastructural studies. Chemical fixation was the primary method of fixing tissue for transmission electron microscopy for many years, as it provides adequate preservation of the morphology of cells and organelles. High pressure freezing (HPF) and freeze substitution (FS) is a newer alternative method that rapidly freezes non-cryoprotected samples that are then slowly heated in the FS medium, allowing penetration of the tissue to insure adequate fixation. This study addresses several issues related to tissue preservation for electron microscopy. Using mice liver tissue as model the difference between samples fixed chemically or with HPF immediately after excision, or stored before chemical or HPF fixation were tested with specific focus on the nuclear membrane. Findings are that immediate HPF is the method of choice compared to chemical fixation. Of the chemical fixatives, immediate fixation with 2.5% glutaraldehyde (GA)/formaldehyde (FA) is the best in preserving membrane morphology, 2.5% GA can be used as alternative for stored and then chemically processed samples, with 10% formalin being suitable as a storage medium only if followed by HPF fixation. Overall, storage leads to lower ultrastructural preservation, but HPF with FS can minimize these artifacts relative to other processing protocols.http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1097-0029hb201

Profile of stress factors associated with mental disorders in children and adolescents referred for evaluation and treatment to the Free State Psychiatric Complex, 2007

Introduction. South African children and adolescents face serious challenges. Over the past decades children have been exposed to rapid and stressful changes in their environment, including increased crime and violence. Aim of study. The aim of the study was to determine the profile of stress factors leading to mental disorders in children and adolescents referred to the Child and Adolescent Unit at the Free State Psychiatric Complex, Bloemfontein, from January 2006 to December 2007. Methods. A total of 669 children (0 - 12 years) and adolescents (13 - 18 years) referred to the unit for evaluation and treatment were included in the study. Results. Thirty per cent were diagnosed with attention deficit and disruptive behaviour disorders, followed by major depressive disorders (22.7%), anxiety disorders (18.5%), conduct disorders (16.1%), mild mental retardation (15.7%), adjustment disorders (9.6%), elimination disorders (8.8%), developmental disorders (7.6%) and bereavement (7.0%). Social stressors were identified in 64.1% of participants, and psychological stressors in 19%. Conclusions. Stress plays an important role in the lives of children and adolescents, which could lead to emotional problems if not well managed. The functioning of children and adolescents should be monitored continuously. Schools are in a favourable position to identify stressors affecting children and adolescents. Educators therefore need training and opportunities to consult on mental health matters. Furthermore, religious organisations should be enlisted to identify stressors manifesting as spiritual dysfunction. School health services can play a role in the recognition of biological stressors such as epilepsy, pregnancy, enuresis, illness, speech problems and sensory dysfunction